1. Field of the Invention
The present invention relates to the treatment of an impure phosphoric acid, and, more especially, relates to the treatment of raw, wet process phosphoric acid prior to the conventional liquid-liquid extraction thereof.
2. Description of the Prior Art
It is well known to this art that the digestion of phosphate rock with a strong mineral acid, such as sulfuric acid, yields an aqueous suspension of phosphoric acid, which, after the filtration therefrom of the gypsum concomitantly formed therein provides a phosphoric acid solution containing numerous and varied impurities. These impurities result from the individual impurities or contaminants contained in the beginning rock and solubilized during the attack of the sulfuric acid, or remaining in the phosphoric acid, after filtration of the gypsum, as fine particles in suspension. Such impurities comprise mineral materials, such as colloidal silica materials, compounds of aluminum, iron, calcium and magnesium, fluorides, sulfates, compounds of uranium and of the rare earth metals, various organic substances, such as humic acids contained in the beginning rock and organic materials intentionally introduced during the pretreatment of the mineral or during the digestion thereof, and generally comprising fatty acids, amines or hydrocarbons. Certain of these impurities are in a state of saturation or supersaturation so that the solution only rarely attains a stable state.
The subsequent typical treatments of the raw solution of phosphoric acid noted hereinabove by means of a combination of solvent extractions and liquid-liquid reextraction is fraught with numerous complexities and difficulties. During this subsequent treatment, the presence of the solvent, together with the agitation of the aqueous and organic phases in a mixer-decanter train accelerate the precipitation of the impurities in the saturated or supersaturated state, which then accumulate at the base of the apparatus. On the other hand, an intermediate colored organic phase appears at the interface of the aqueous and organic phases, containing the organic impurities of the raw acid. Upon the start-up of an extraction unit, these phenomena first manifest themselves in the inlet mixer-decanter of the process battery or train and are then propagated into the following units, necessitating that the battery or apparatus train be periodically shut down for cleaning. It is impossible under these conditions to operate the apparatus in a continuous and industrially profitable manner. Furthermore, it appears that the intermediate colored organic phase solubilizes the extraction agent typically introduced into the organic phase and thereby modifies the partition coefficients of the valuable metallic impurities. A great need exists in this industry, therefore, for a process for the treatment of crude, wet process phosphoric acid in order to first remove therefrom the major part of precipitable mineral and organic impurities from the acid and to facilitate additional downstream treatment. Generally compare French Pat. Nos. 1,336,816, 1,564,461 and 2,255,376, and U.S. Pat. No. 3,186,793.